Electrotherapy circuit having controlled peak current

ABSTRACT

An electrotherapy circuit administers to a patient a current waveform. The electrotherapy circuit includes a charge storage device, at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, a sensor that senses a patient-dependent electrical parameter (such as a patient impedance sensor), and a control circuit. The control circuit is connected to the sensor and the charge storage device and controls discharge of the charge storage device through the electrodes, based on the patient-dependent electrical parameter (such as the patient impedance) as sensed by the sensor. The discharge is controlled in a manner so as to reduce the dependence of peak discharge current on the electrical parameter (such as patient impedance) for a given amount of charge stored by the charge storage device.

BACKGROUND OF THE INVENTION

This invention relates to electrotherapy circuits and more particularlyrelates to external defibrillators that apply defibrillation shocks to apatient's heart through electrodes placed externally on the patient'sbody or externally on the patient's heart during surgery.

Normally, electrochemical activity within a human heart causes theorgan's muscle fibers to contract and relax in a synchronized manner.This synchronized action of the heart's musculature results in theeffective pumping of blood from the ventricles to the body's vitalorgans. In the case of ventricular fibrillation (VF), however, abnormalelectrical activity within the heart causes the individual muscle fibersto contract in an unsynchronized and chaotic way. As a result of thisloss of synchronization, the heart loses its ability to effectively pumpblood.

Defibrillators produce a large current pulse that disrupts the chaoticelectrical activity of the heart associated with ventricularfibrillation and provide the heart's electro-chemical system with theopportunity to resynchronize itself. Once organized electrical activityis restored, synchronized muscle contractions usually follow, leading tothe restoration of effective cardiac pumping.

The current required for effective defibrillation is dependent upon theparticular shape of the current waveform, including its amplitude,duration, shape (i.e., sine, damped sine, square, exponential decay),and whether the current waveform has a single polarity (monophasic) orhas both positive and negative polarity (biphasic). It has beensuggested that large defibrillation currents may cause damage to cardiactissue, however.

It is known to construct an external defibrillator that can sensepatient impedance and can set the durations of the first and secondphases of a biphasic waveform as a function of the patient impedance. Anexample of such a defibrillator is described in PCT Patent PublicationNo. WO 95/05215. Fain et al., U.S. Pat. No. 5,230,336 discloses a methodof setting pulse widths of monophasic and biphasic defibrillationwaveforms based on measured patient impedance. Kerber et al., "AdvancePrediction of Transthoracic Impedance in Human Defibrillation andCardioversion: Importance of Impedance in Determining the Success ofLow-Energy Shocks," 1984, discloses a method of selecting the energy ofdefibrillation shocks based on patient impedance measured using ahigh-frequency signal.

It is also known to construct a defibrillator with a safety resistor inthe defibrillation path (PCT Patent Publication No. WO 95/05215). Beforeapplication of a defibrillation waveform to a patient, a test pulse ispassed through the safety resistor while a current sensor monitors thecurrent. If the sensed current is less than a safety thresholdrepresentative of a short circuit, the safety resistor is removed andthe defibrillation waveform is applied to the patient.

It is known, in an implantable defibrillator, to use a biphasic waveformhaving a first phase consisting of multiple truncated decayingexponentials that form a sawtooth approximation of a rectilinear shape(Kroll, U.S. Pat. No. 5,199,429). This is accomplished by charging a setof energy storage capacitors and then successively allowing individualcapacitors to discharge during the first phase, thereby creating thesawtooth pattern in the output current of the circuit. A more recentpatent, Kroll, U.S. Pat. No. 5,514,160, describes a biphasic waveform,in an implantable defibrillator, having a rectilinear-shaped first phasecreated by placing a MOSFET current limiter in the defibrillation path.This patent states that the grossly non-linear current limiter lookslike a small and declining resistance to the capacitor. Also, Schuder etal., "Transthoracic Ventricular Defibrillation in the 100 kg Calf withSymmetrical One-Cycle Bidirectional Rectangular Wave Stimuli" describesthe use of biphasic waveforms having rectilinear first and second phasesto reverse ventricular fibrillation in calves. Stroetmann et al., U.S.Pat. No. 5,350,403, discloses a waveform having a sawtooth ripple thatis formed by periodically interrupting a non-continuous discharge of acharging circuit.

SUMMARY OF THE INVENTION

The invention features an electrotherapy circuit for administering to apatient a current waveform such as a defibrillation waveform. Theelectrotherapy circuit includes a charge storage device, at least twodischarge electrodes connected by electrical circuitry to opposite polesof the charge storage device, a sensor that senses a patient-dependentelectrical parameter (such as a patient impedance sensor), and a controlcircuit. The control circuit is connected to the sensor and the chargestorage device and controls discharge of the charge storage devicethrough the electrodes, based on the patient-dependent electricalparameter (such as the patient impedance) as sensed by the sensor. Thedischarge is controlled in a manner so as to reduce the dependence ofpeak discharge current on the electrical parameter (such as patientimpedance) for a given amount of charge stored by the charge storagedevice.

By controlling the discharge of the charge storage device based on thesensed patient impedance, it is possible to limit the difference in thepeak current that passes through a low-impedance patient as comparedwith a high-impedance patient. Thus, the current is made more constantover a range of patient impedances, and the electrotherapy circuitprovides effective defibrillation while maintaining controlled currentlevels to reduce any possibility of damage to heart, skin, and muscletissue.

In preferred embodiments the control circuit controls the discharge ofthe charge storage device by controlling the resistance of a resistivecircuit connected between the charge storage device and one of theelectrodes. The resistive circuit includes a set of resistors connectedtogether in series.

In preferred embodiments the control circuit decides, based on thepatient impedance sensed during an initial sensing pulse portion of thedischarge of the charge storage device, how many (if any) resistors toinclude in the defibrillation path at the beginning of a therapeuticdischarge portion of the discharge of the charge storage device (e.g.,at the beginning of a biphasic defibrillation waveform). This may mean,depending on the sensed patient impedance, that the current level stepsup from the sensing pulse to the beginning of the biphasicdefibrillation waveform. Once the biphasic defibrillation waveformbegins, the resistors that are present in the defibrillation path aresuccessively shorted out, thereby creating a sawtooth approximation to arectilinear shape in the output current (output decays and then jumps upevery time a resistor is shorted out) and compensating for thedecreasing capacitor voltage.

This provides an improved, low-cost way of creating a biphasic waveformhaving a rectilinear first phase. Resistors are relatively inexpensiveas compared with capacitors, and a total of N steps in resistance valuescan be obtained with log₂ N resistors, as opposed to N capacitors,simply by connecting the resistors in series in a binary sequence(1-2-4-etc.). Because resistors are used instead of capacitors, nocircuitry is required to equalize voltages on capacitors upon rechargeor to prevent reversal of voltages on capacitors.

Certain embodiments include a variable resistor stage that tends tosmooth out the sawtooth pattern. The variable resistor stage is acircuit that is reset to its maximum resistance value every time one ofthe fixed-value resistors is shorted out and then decreases to zero overthe time interval before the next resistance step reduction.

Another advantage of the invention is that the resistors in thedefibrillation path inherently protect against possible short circuits.

We believe that the use of a waveform having a substantially rectilinearpositive phase tends to provide a lower threshold of average currentrequired for effective defibrillation, and tends to avoid damaging thepatient's tissue even if the total energy applied to the patient isrelatively high. We also believe that any sawtooth ripple in eitherphase of the waveform should preferably have a height less than aboutone-quarter of the average height of the phase, and more preferably lessthan about one-sixth of the average height of the phase, in order tofurther minimize the threshold of average current required for effectivedefibrillation and the possibility of damaging the patient's tissue.

The impedance of a patient when a large direct current is passingthrough the patient is different from the impedance of the patient whena small current is passing through the patient or when an alternatingcurrent is passing through the patient. We believe that the currentlevel of the sensing portion should always be at least one-third, andmore preferably one-half, of the current level at the beginning of thetherapeutic discharge portion in order to ensure detection of a patientimpedance that is similar to the impedance of the patient during thetherapeutic discharge portion.

Preferably, the discharge of the charge storage device occurs withoutrecharging of the charge storage device between the sensing pulseportion and the therapeutic discharge portion of the current waveform.Thus, it is possible to apply paddles to the chest of the patient (orapply hand-held spoons directly to the patient's heart during open heartsurgery) and immediately discharge the sensing pulse and then dischargethe biphasic defibrillation waveform immediately after the sensingpulse. This is particularly important because the patient (or thepatient's heart) may move and because it is difficult for a practitionerto apply a constant force to the patient's skin (or the patient'sheart).

Numerous other features, objects, and advantages of the invention willbecome apparent from the following detailed description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a current waveform produced by a electrotherapycircuit according to the invention.

FIG. 2 is a diagram of the key elements of electrotherapy circuitaccording to the invention.

FIG. 3 is a schematic diagram of the series-connected resistor circuitshown in the electrotherapy circuit of FIG. 2.

FIG. 4 is a schematic diagram of the H-bridge circuit shown in theelectrotherapy circuit of FIG. 2.

FIG. 5 is a schematic diagram of the variable resistor shown in theelectrotherapy circuit of FIG. 2.

FIGS. 6-9 are diagrams of current waveforms produced by anelectrotherapy circuit according to the invention based on differentmeasured patient impedances.

FIG. 10 is a set of schedules of the resistance values used forgenerating the waveforms shown in FIGS. 6-10.

FIG. 11 is a table of waveform parameters for various patient impedancesin a "normal" mode of operation and a "high-energy" mode of operation ofan electrotherapy circuit according to the invention.

DETAILED DESCRIPTION

With reference to FIG. 1, in operation of an external defibrillatoraccording to the invention, the biphasic current waveform begins with aninitial "sensing pulse" 10, which has insufficient energy for performingtherapy. The sensing pulse is integral with, i.e., immediately followedby, a biphasic defibrillation waveform having sufficient energy fordefibrillating the patient's heart. The biphasic defibrillation waveformincludes a six-millisecond, generally rectilinear positive phase 12having a sawtooth ripple 14, which is in turn followed by a fourmillisecond negative phase 16 that decays exponentially until thewaveform is truncated. As used herein, the term "rectilinear" meanshaving a straight line, regardless of whether the straight line is flator slightly tilted. The current waveform decreases through a series ofsteps 18 from the end of the positive phase to the beginning of negativephase, one of the steps being at the zero crossing. Note that forpurposes of clarity this 0.1-millisecond transition is not drawn toscale in FIG. 1; if drawn to scale the duration of this transition wouldbe much shorter than shown in FIG. 1.

We believe that a biphasic defibrillation waveform having a positiverectilinear pulse of 6 milliseconds duration followed by 0.1-millisecondtransition and a 4 millisecond negative pulse having an initialamplitude equal to the final amplitude of the positive pulse is anespecially effective waveform for defibrillation. The negative pulsedoes not need to be rectilinear.

The basic circuitry for producing the biphasic waveform is shown in FIG.2. A storage capacitor 20 (115 μF) is charged to a maximum of 2200 voltsby a charging circuit 22 while relays 26 and 28 and the H-bridge areopen, and then the electric charge stored in storage capacitor 20 isallowed to pass through electrodes 21 and 23 and the body of a patient24. In particular, relay switches 17 and 19 are opened, and then relayswitches 26 and 28 are closed. Then, electronic switches 30, 32, 34, and36 of H-bridge 48 are closed to allow the electric current to passthrough the patient's body in one direction, after which electronicH-bridge switches 30, 32, 34, and 36 are opened and H-bridge switches38, 40, 42, and 44 are closed to allow the electric current to passthrough the patient's body in the opposite direction. Electronicswitches 30-44 are controlled by signals from respective opto-isolators,which are in turn controlled by signals from a microprocessor 46, oralternatively a hard-wired processor circuit. Relay switches 26, and 28,which are also controlled by microprocessor 46, isolate patient 24 fromleakage currents of bridge switches 30-44, which may be about 500micro-amps. Relay switches 26 and 28 may be relatively inexpensivebecause they do not have to "hot switch" the current pulse. They close afew milliseconds before H-bridge 48 is "fired" by closure of some of theH-bridge switches.

Electrodes 21 and 23 may be standard defibrillation electrodes havingflat surfaces that adhere to the chest of the patient, but they mayalternatively be hand-held paddles that are applied to the chest of thepatient or hand-held spoons that are applied directly to the patient'sheart during open heart surgery. Storage capacitor 20 may be a singlecapacitor or a set of series-connected or parallel-connected capacitors.

A resistive circuit 50 that includes series-connected resistors 52, 54,and 56 is provided in the current path, each of the resistors beingconnected in parallel with a shorting switch 58, 60, and 62 controlledby microprocessor 46. The resistors are of unequal value, stepped in abinary sequence to yield 2^(n) possible resistances where n is thenumber of resistors. During the initial "sensing pulse," when H-bridgeswitches 30, 32, 34, and 36 are closed, all of the resistor-shortingswitches 58, 60, and 62 are in an open state so that the current passesthrough all of the resistors in series. Current-sensing transformer 64senses the current passing through the patient 24, from whichmicroprocessor 46 determines the resistance of the patient 24.

The initial sensing pulse is integral with, i.e., immediately followedby, a biphasic defibrillation waveform, and no re-charging of storagecapacitor 20 occurs between the initial sensing pulse and the biphasicdefibrillation waveform.

If the patient resistance sensed during the initial sensing pulse islow, all of the resistor-shorting switches 58, 60, and 62 are left openat the end of the sensing pulse so that all of the resistors 52, 54, and56 remain in the current path (the resistors are then successivelyshorted out during the positive phase of the biphasic defibrillationwaveform in the manner described below in order to approximate arectilinear positive phase). Thus, the current at the beginning of thepositive first phase 12 of the biphasic defibrillation waveform is thesame as the current during sensing pulse 10. If the patient resistancesensed during the sensing pulse is high, some or all of theresistor-shorting switches 58, 60, and 62 are closed at the end of thesensing pulse, thereby shorting out some or all of the resistors. Thiscauses an upward jump in current at the end of the sensing pulse asshown in the waveform in FIG. 1.

Thus, immediately after the sensing pulse, the biphasic defibrillationwaveform has an initial discharge current that is controlled bymicroprocessor 46 based on the patient impedance sensed bycurrent-sensing transformer 64.

The current level of the sensing pulse is always at least 50 percent ofthe current level at the beginning of positive first phase 12, and thesensing pulse, like the defibrillation pulse, is of course adirect-current pulse.

By appropriately selecting the number of resistors that remain in thecurrent path, microprocessor 46 reduces (but does not eliminate) thedependence of peak discharge current on patient impedance, for a givenamount of charge stored by the charge storage device. For a patientresistance of 15 ohms the peak current is about 25 amps, whereas for apatient resistance of 125 ohms the peak current is about 12.5 amps (atypical patient impedance is about 75 ohms).

During the positive phase of the biphasic waveform some or all of theresistors 52, 54, and 56 that remain in series with the patient 24 aresuccessively shorted out. Every time one of the resistors is shortedout, an upward jump in current occurs in the waveform, thereby resultingin the sawtooth ripple shown in the waveform of FIG. 1. The ripple tendsto be greatest at the end of the rectilinear phase because the timeconstant of decay (RC) is shorter at the end of the phase than at thebeginning of the phase. Of course, if all of the resistors have alreadybeen shorted out immediately after the end of the sensing pulse, thepositive phase of the biphasic waveform simply decays exponentiallyuntil the waveform switches to the negative phase.

As is shown in FIG. 1, at the end of the positive phase, the currentwaveform decreases through a series of rapid steps from the end of thepositive phase to the beginning of negative phase, one of the stepsbeing at the zero crossing. Microprocessor 46 accomplishes this by 1)successively increasing the resistance of resistive circuit 50 in fixedincrements through manipulation of resistor-shorting switches 58, 60,and 62, then 2) opening all of the switches in H-bridge 48 to bring thecurrent waveform down to the zero crossing, then 3) reversing thepolarity of the current waveform by closing the H-bridge switches thathad previously been open in the positive phase of the current waveform,and then 4) successively decreasing the resistance of resistance circuit50 in fixed increments through manipulation of resistor-shortingswitches 58, 60, and 62 until the resistance of resistance circuit 50 isthe same as it was at the end of the positive phase.

In one embodiment a variable resistor 66 is provided in series with theother resistors 52, 54, and 56 to reduce the sawtooth ripple. Every timeone of the fixed-value resistors 52, 54, or 56 is shorted out, theresistance of variable resistor 66 automatically jumps to a high valueand then decreases until the next fixed-value resistor is shorted out.This tends, to some extent, to smooth out the height of the sawtoothripple from about 3 amps to about 0.1 to 0.2 amps, and reduces the needfor smaller increments of the fixed-value resistors (i.e., it reducesthe need for additional fixed-value resistor stages).

The rectilinear phase may exhibit a degree of tilt, either slightly up,or slightly down. This occurs because of the "graininess" of the steps,because patient impedance may change during the waveform, and because ofinherent inaccuracies of circuit elements. For example, with respect tograininess of the steps, calculations might show that, for a 50-ohmpatient, the optimal resistance required at the end of the rectilinearphase is 14 ohms, in which case we must choose between 10 or 20 ohmsbased on the available fixed-value resistors. If we choose 10 ohms, an"error" of 4 ohms would result at the end of the rectilinear phase, andthe current would rise by about 6 or 7 percent (14-10)/(50+14)! by theend of the phase. Thus, a 15 amp rectilinear pulse would rise from 15amps to 16 amps over the rectilinear phase. If it were considereddesirable to change this rise to a droop, the microprocessor couldeasily accommodate such a change. In general, we believe it is desirableto avoid tilt greater than about 20 percent in order to avoid passage ofexcessive current through the patient's body at the high end of thetilt.

The choices of capacitor (115 μF) and voltage (2200 volts) are based onthe desired current requirements and allowable droop during the negativephase. The capacitor stores the minimum energy required to meet thedelivered charge requirements (i.e., the charge required to produce thedesired current waveform having the desired duration).

The switches in the left-hand side of H-bridge 48 can be tested byclosing switches 17 and 19, opening switches 26 and 28, closing switches30 and 32, then after a short time closing switches 42 and 44, thenafter a short time opening switches 30 and 32, and then after a shorttime opening switches 42 and 44. If the switches are working properly,current-sensing transformer 64 will sense the passage of current whenall four switches are closed, and will sense no current when switches 30and 32 or switches 42 and 44 are open. Otherwise, current-sensingtransformer 64 will detect the possible presence of a short circuit oran open circuit. Similarly, the switches in the right-hand side ofH-bridge 48 can be tested by closing switches 38 and 40, then after ashort time closing switches 34 and 36, then after a short time openingswitches 38 and 40, and then after a short time opening switches 34 and36. This valuable safety test does not require current to pass throughthe patient, due to the placement of current-sensing transformer 64outside the legs of H-bridge 48.

Microprocessor 46 easily accommodates a complex environment andfunctions in harmony with various controls, interlocks, and safetyfeatures of the electrotherapy system. In addition to performing thefunctions described herein, the microprocessor may operate a stripchart, a pacer, an ecg monitor, etc. In the event that additionalresearch should show that the characteristics of current pulses shouldbe different from those described herein, the microprocessor can bere-programmed to alter the current waveforms applied to the patient. Forexample, the microprocessor could accommodate a waveform change toproduce a rising or falling rectilinear ramp voltage with time, or awaveform having a negative phase amplitude less than (or greater than)the positive phase amplitude. Of course, the storage capacitor must haveenough stored charge to support the required output.

In an alternative embodiment the negative phase of the current waveformis substantially rectilinear, rather than exponentially decaying, andthe techniques described above for providing a substantially rectilinearpositive phase would be extended to produce the substantiallyrectilinear negative phase. Such a substantially rectilinear negativephase would require the use of a higher capacitance and voltage andhigher-rated switching devices than those employed in the circuit ofFIG. 2 (for a given initial current value of the negative phase).

Referring to FIG. 3, the resistive circuit 50 of FIG. 2 includesresistors 52 (10 ohms), 54 (two 10-ohm resistors), and 56 (four 10-ohmresistors) and IGBT shorting switches 58, 60, and 62. Alternatively,other semiconductor switching devices may be used. The resistor stringis designed to switch in 10-ohm steps. This allows for a maximumresistance of 80-ohms (including the 10-ohm variable resistor), whichmakes it possible to limit patient current to 21.5 amps for a 15-ohmpatient resistance (the current pulse would be 25.6 amps in the event ofa short circuit between the electrodes).

The values of the resistors, as well as the 115 μF value of the storagecapacitor and the capacitor voltage of 2200 volts, are determined by thecurrent required to be delivered into the patient load (about 12.5-25amps) and the range of the patient load (e.g., 125 ohms-15 ohms). TheIGBT shorting switches are switched on and off by means of opto-isolatorcircuits 68, 70, and 72 controlled by the microprocessor. Referring toFIG. 4, H-bridge 48 of FIG. 2 includes IGBT switches 30-44 similarlyswitched on and off by means of opto-isolator circuits 74, 76, 78, and80 controlled by the microprocessor. Alternatively, switches 30-44 maybe other types of semiconductor switching devices. Only oneopto-isolator is provided to control each pair of switches in each armof the H-bridge.

Referring to FIG. 5, variable resistor 66 of FIG. 2 includes resistor 82connected between resistive circuit 50 and storage capacitor 20. Theeffective resistance of variable resistor 66 is controlled by thecircuitry connected in parallel with resistor 82, through which some ofthe current from storage capacitor 20 to resistive circuit 50 can pass.

In particular, whenever the microprocessor shorts out one of thefixed-value resistors in resistive circuit 50, it also shorts capacitor84. This causes transistor 86 to turn on, which pulls the gate of FET orIGBT transistor 88 to ground, thereby turning transistor 88 off. Becausetransistor 88 is turned off, all of the current from storage capacitor20 to resistive circuit 50 passes through resistor 82.

Capacitor 84 then begins to charge linearly because of the currentsource in the collector of transistor 86. This causes the voltage at thedrain/collector of transistor 88 to increase linearly, which causes thecurrent in transistor 88 to increase linearly. When the current intransistor 88 increases, the current passing through resistor 82decreases, thereby reducing the voltage across resistor 82 and thereforereducing the effective resistance of variable resistor 66.

The electrotherapy circuit can be operated in either a "normal" mode ofoperation or a "high-energy" mode of operation. These two modes ofoperation are identical when the sensed patient impedance is below 40ohms. If the sensed patient impedance is above 40 ohms, however, themicroprocessor selects an initial resistance value of theseries-connected resistors (after the sensing pulse) that depends on themode of operation. In particular, in the "high-energy" mode of operationthe microprocessor selects a lower initial resistance than in the"normal" mode of operation. Thus, more energy will be delivered to thepatient in the "high-energy" mode of operation than in the "normal" modeof operation. A practitioner may try to defibrillate in the "normal"mode, then switch to the "high-energy" mode if unsuccessful.

In the "high-energy" mode of operation of the circuit, if the sensedpatient impedance is sufficiently high (above 85 ohms) all of theresistor-shorting switches are closed after the initial "sensing pulse,"thereby shorting out all of the series-connected resistors. This causesan upward jump at the end of the "sensing pulse," after which thepositive and negative phases of the biphasic waveform both decayexponentially.

Referring to the table of FIG. 10 and the waveforms of FIGS. 6-9, whichcorrespond to certain schedules in the table of FIG. 10, themicroprocessor schedules the resistance values of the series-connectedresistors based on the measured patient impedance, in a manner such thatthe stepwise resistance decrease of the series-connected resistors overthe course of the rectilinear phase matches the decrease in voltage ofthe storage capacitor. For the sake of simplicity, the initial sensingpulse and the series of steps between the end of the positive phase andthe beginning of negative phase have been omitted from FIGS. 6-9, and itis assumed the variable resistor discussed above is not used. FIGS. 6-9are all examples of the "high-energy" mode. FIG. 6, which corresponds toSchedule 3A in FIG. 10, is based on a patient impedance of 50 ohms, inwhich case the microprocessor selects an initial series-connectedresistance of 30 ohms and a residual series-connected resistance of 0ohms at the end of the positive phase. The total energy delivered to thepatient is about 182 joules. FIG. 7, corresponding to Schedule 4A, isbased on a patient impedance of 75 ohms, an initial resistance of 10ohms, a residual resistance of 0 ohms, and an energy of 222 joules. FIG.8, corresponding to Schedule 5A, is based on a patient impedance of 100ohms, an initial resistance of 0 ohms, and a residual resistance of 0ohms, and an energy of 217 joules. FIG. 9, corresponding to Schedule 5A,is based on a patient impedance of 125 ohms, an initial resistance of 40ohms, a residual resistance of 0 ohms, and an energy of 199 joules.

FIG. 11 includes a table, corresponding to the "normal" mode ofoperation, that identifies, as a function of the patient impedance, thepositive-phase current (in amps), ripple (in amps, assuming the variableresistor is not used), tilt of the negative phase (expressed as apercentage of the initial current value of the negative phase), totaldelivered energy (in joules), and the deviation of the total deliveredenergy from the normal mode's "rating" of 150 joules. FIG. 11 alsoincludes a similar table for the "high-energy" mode of operation,identifying positive-phase current, tilt of the positive phase (based ona straight-line average through the ripples), ripple, tilt of thenegative phase, total delivered energy, and the deviation of the totaldelivered energy from the "rating" of 170 joules.

In both the high-energy and normal modes described above, the storagecapacitor is charged to its maximum voltage of 2200 volts. Other modesof operation can be developed in which the storage capacitor is chargedto a lesser voltage, or in which different resistance schedules areused.

There have been described novel and improved electrotherapy circuits andtechniques for using them. It is evident that those skilled in the artmay now make numerous uses and modifications of and departures from thespecific embodiment described herein without departing from theinventive concept. For example, the techniques described herein can beused in connection with implantable defibrillators rather than externaldefibrillators or in connection with electrotherapy circuits other thandefibrillator circuits or even circuits that perform functions otherthan electrotherapy.

What is claimed is:
 1. An electrotherapy circuit for administering to apatient a current waveform, comprising:a charge storage device; at leasttwo discharge electrodes connected by electrical circuitry to oppositepoles of the charge storage device; a sensor that senses apatient-dependent electrical parameter; and a control circuit, connectedto the sensor and the charge storage device, that controls discharge ofthe charge storage device through the electrodes, during the dischargeof the charge storage device, based on the patient-dependent electricalparameter as sensed by the sensor, in a manner so as to reduce thedependence of peak discharge current on the electrical parameter for agiven amount of charge stored by the charge storage device.
 2. Theelectrotherapy circuit of claim 1 wherein the patient-dependentelectrical parameter comprises impedance of the patient.
 3. Theelectrotherapy circuit of claim 1 wherein the sensor is a currentsensor.
 4. The electrotherapy circuit of claim 3 wherein the sensor is acurrent sense transformer.
 5. The electrotherapy circuit of claim 1wherein the peak discharge current if the impedance of the patient is 15ohms is no more than about twice the peak discharge current if theimpedance of the patient is 125 ohms.
 6. The electrotherapy circuit ofclaim 1 further comprising at least one switch connected between thecharge storage device and one of the electrodes that, when closed,creates a closed circuit for flow of current from the charge storagedevice to the electrodes.
 7. The electrotherapy circuit of claim 6wherein the at least one switch comprises a plurality of switchesarranged as an H-bridge.
 8. The electrotherapy circuit of claim 1wherein the current waveform comprises a first phase and a second phasehaving a polarity opposite to the first phase.
 9. The electrotherapycircuit of claim 1 wherein the charge storage device comprises at leastone capacitor.
 10. The electrotherapy circuit of claim 9 wherein thecharge storage device is a single capacitor.
 11. The electrotherapycircuit of claim 1 wherein the control circuit comprises amicroprocessor.
 12. The electrotherapy circuit of claim 1 wherein thecontrol circuit is hard-wired.
 13. The electrotherapy circuit of claim 1wherein the current waveform comprises a defibrillation pulse.
 14. Theelectrotherapy circuit of claim 1 wherein the electrodes arenon-implanted.
 15. An electrotherapy circuit for administering to apatient a current waveform, comprising:a charge storage device; at leasttwo discharge electrodes connected by electrical circuitry to oppositepoles of the charge storage device; a sensor that senses apatient-dependent electrical parameter; a control circuit, connected tothe sensor and the charge storage device, that controls discharge of thecharge storage device through the electrodes, based on thepatient-dependent electrical parameter as sensed by the sensor, in amanner so as to reduce the dependence of peak discharge current on theelectrical parameter for a given amount of charge stored by the chargestorage device; and a variable impedance connected between the chargestorage device and one of the electrodes, the control circuit beingconnected to the variable impedance and controlling discharge of thecharge storage device by controlling the variable impedance duringdischarge of the charge storage device.
 16. The electrotherapy circuitof claim 15 wherein the control circuit causes selects the impedance ofthe variable impedance inversely with respect to the impedance of thepatient as sensed by the sensor.
 17. The electrotherapy circuit of claim4 wherein the discharge of the charge storage device comprises a currentwaveform having a sensing pulse portion during which the sensor sensesthe impedance of the patient, and a therapeutic discharge portion duringwhich the control circuit selects the impedance of the variableimpedance inversely with respect to the impedance of the patient assensed by the sensor during the sensing pulse portion.
 18. Theelectrotherapy circuit of claim 17 wherein the therapeutic dischargeportion of the current waveform is rectilinear.
 19. The electrotherapycircuit of claim 3 wherein the variable impedance comprises a variableresistive circuit.
 20. The electrotherapy circuit of claim 19 whereinthe resistive circuit comprises a plurality of discrete resistors. 21.The electrotherapy circuit of claim 20 further comprising a switchingcircuit connected to the plurality of resistors that selectivelyprovides at least one path for flow of electric current from the chargestorage device through a subset of the plurality of resistors to one ofthe discharge electrodes.
 22. The electrotherapy circuit of claim 21wherein the control circuit controls the switching circuit to select thesubset of the resistors through which the electric current flows, thecontrol circuit selecting different subsets of the resistors duringdifferent portions of discharge of the charge storage device so as toproduce a rectilinear current waveform.
 23. The electrotherapy circuitof claim 21 wherein the resistors are connected together in series. 24.The electrotherapy circuit of claim 23 wherein the switching circuitselectively provides the path for flow of electric current by shortingout resistors not in the subset through which the path extends.
 25. Theelectrotherapy circuit of claim 20 wherein the resistors are stepped ina binary sequence.
 26. A method of forming an electrotherapy currentwaveform, comprising the steps of:charging a charge storage device;sensing a patient-dependent electrical parameter; discharging the chargestorage device through at least two discharge electrodes connected byelectrical circuitry to opposite poles of the charge storage device; andcontrolling discharge of the charge storage device through theelectrodes, during the discharge of the charge storage device, based onthe patient-dependent electrical parameter, in a manner so as to reducedependence of peak discharge current on electrical parameter for a givenamount of charge stored by the charge storage device.
 27. The method ofclaim 26 wherein the patient-dependent electrical parameter comprisesimpedance of the patient.
 28. The method of claim 26 wherein the peakdischarge current if the impedance of the patient is 15 ohms is no morethan about twice the peak discharge current if the impedance of thepatient is 125 ohms.
 29. The method of claim 26 wherein the currentwaveform comprises a first phase and a second phase having a polarityopposite to the first phase.
 30. The method of claim 26 wherein thecurrent waveform comprises a defibrillation pulse.
 31. The method ofclaim 26 further comprising the step of applying the electrodesexternally to the patient prior to discharging the charge storagedevice.
 32. The method of claim 31 wherein the electrodes are externallyapplied directly to the patient's heart during surgery.
 33. A method offorming an electrotherapy current waveform, comprising the stepsof:charging a charge storage device; sensing a patient-dependentelectrical parameter; discharging the charge storage device through atleast two discharge electrodes connected by electrical circuitry toopposite poles of the charge storage device; and controlling dischargeof the charge storage device through the electrodes, based on thepatient-dependent electrical parameter, in a manner so as to reducedependence of peak discharge current on electrical parameter for a givenamount of charge stored by the charge storage device; wherein thestorage device is discharged through a variable impedance connectedbetween the charge storage device and one of the electrodes; and whereinthe step of controlling discharge of the charge storage device comprisescontrolling the variable impedance during discharge of the chargestorage device based on the patient-dependent electrical parameter. 34.The method of claim 33 wherein the step of controlling the variableimpedance comprises setting the variable impedance inversely withrespect to the sensed impedance of the patient.
 35. The method of claim34 wherein:the discharge of the charge storage device comprises acurrent waveform having a sensing pulse portion and a therapeuticdischarge portion; the step of sensing the impedance of the patient isperformed during the sensing pulse portion; and the step of controllingthe variable impedance comprises, during the therapeutic dischargeportion, setting the variable impedance inversely with respect to theimpedance of the patient as sensed during the sensing pulse portion. 36.The method of claim 33 wherein the variable impedance comprises aplurality of discrete resistors and the step of controlling the variableimpedance comprises selectively providing at least one path for flow ofelectric current from the charge storage device through a subset of theplurality of resistors to one of the discharge electrodes.
 37. Themethod of claim 35 wherein the step of providing the path for flow ofelectric current comprises selecting different subsets of the resistorsduring different portions of discharge of the charge storage device soas to produce a rectilinear current waveform.
 38. The method of claim 35wherein the resistors are connected together in series and the step ofproviding the path for flow of electric current comprises shorting outresistors not in the subset through which the path extends.